Geochemical conditions for the preservation of recent aragonite-rich sediments in Mediterranean karstic marine lakes (Mljet Island, Adriatic Sea, Croatia)
Sonja Lojen A D , Ivan Sondi B and Mladen Juračić CA Jožef Stefan Institute, 1000 Ljubljana, Slovenia.
B Ruđer Bošković Institute, 10 000 Zagreb, Croatia.
C Department of Geology, Faculty of Science, University of Zagreb, 10 000 Zagreb, Croatia.
D Corresponding author. Email: sonja.lojen@ijs.si
Marine and Freshwater Research 61(1) 119-128 https://doi.org/10.1071/MF09034
Submitted: 24 February 2009 Accepted: 18 June 2009 Published: 29 January 2010
Abstract
Conditions for the preservation of recent aragonite-rich sediments during early diagenesis in two semi-enclosed Mediterranean karstic seawater lakes on the island of Mljet (Adriatic Sea) were examined. The concentrations and stable isotope compositions of carbonate and sedimentary organic matter, as well as the geochemical parameters in pore water were measured. It was found that the smaller lake (Malo Jezero) receives considerably more terrestrial detritus than the larger lake (Veliko Jezero). A decrease in carbonate δ13C values with depth indicated a rather intensive transfer of organically derived C into the carbonate pool by diagenetic recrystallisation, masking the changes in carbonate δ13C caused by increasing amounts of aragonite. Dissolution of calcite as a result of CO2 released from the decomposition of organic debris and the upward diffusive flux of dissolved inorganic carbon were together responsible for up to 24% of the dissolved inorganic carbon added to the pore water. This indicated locally occurring carbonate dissolution, irrespective of its saturation state in the bulk sediment. Despite the larger input of terrigenous material into Malo Jezero, the carbonate content in the sediment was much higher than in Veliko Jezero, indicating greater authigenic aragonite production. As magnesium calcite accounted for most of the carbonate dissolution, aragonite preservation in the sediment is favoured.
Additional keywords: carbon, nitrogen, sedimentary organic matter, stable isotopes.
Acknowledgements
This study was supported by the Ministry of Science, Higher Education and Technology of Slovenia (program P1-0143), the Ministry of Science, Education and Sports of the Republic of Croatia (Grant No. 098-0982934-2742 and Grant No. 1119-1191152-1169) and by the authority of the ‘Mljet’ National Park. Sincere thanks to Professor V. Pravdić for thoughtful comments. The authors gratefully acknowledge the help of Dr Nevenka Mikac with the chemical analyses, Dr Srečo D. Škapin with the scanning electron microscope analyses and Dr Anthony R. Byrne for linguistic corrections.
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